465 lines
15 KiB
ReStructuredText
465 lines
15 KiB
ReStructuredText
Loops
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=====
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Often you'll want to do many things in one task, such as create a lot of users, install a lot of packages, or
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repeat a polling step until a certain result is reached.
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This chapter is all about how to use loops in playbooks.
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.. contents:: Topics
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.. _standard_loops:
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Standard Loops
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``````````````
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To save some typing, repeated tasks can be written in short-hand like so::
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- name: add several users
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user: name={{ item }} state=present groups=wheel
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with_items:
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- testuser1
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- testuser2
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If you have defined a YAML list in a variables file, or the 'vars' section, you can also do::
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with_items: "{{somelist}}"
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The above would be the equivalent of::
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- name: add user testuser1
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user: name=testuser1 state=present groups=wheel
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- name: add user testuser2
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user: name=testuser2 state=present groups=wheel
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The yum and apt modules use with_items to execute fewer package manager transactions.
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Note that the types of items you iterate over with 'with_items' do not have to be simple lists of strings.
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If you have a list of hashes, you can reference subkeys using things like::
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- name: add several users
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user: name={{ item.name }} state=present groups={{ item.groups }}
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with_items:
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- { name: 'testuser1', groups: 'wheel' }
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- { name: 'testuser2', groups: 'root' }
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Also be aware that when combining `when` with `with_items` (or any other loop statement), the `when` statement is processed separately for each item. See :ref:`the_when_statement` for an example.
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.. _nested_loops:
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Nested Loops
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````````````
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Loops can be nested as well::
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- name: give users access to multiple databases
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mysql_user: name={{ item[0] }} priv={{ item[1] }}.*:ALL append_privs=yes password=foo
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with_nested:
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- [ 'alice', 'bob' ]
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- [ 'clientdb', 'employeedb', 'providerdb' ]
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As with the case of 'with_items' above, you can use previously defined variables.::
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- name: here, 'users' contains the above list of employees
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mysql_user: name={{ item[0] }} priv={{ item[1] }}.*:ALL append_privs=yes password=foo
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with_nested:
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- "{{users}}"
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- [ 'clientdb', 'employeedb', 'providerdb' ]
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.. _looping_over_hashes:
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Looping over Hashes
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```````````````````
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.. versionadded:: 1.5
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Suppose you have the following variable::
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---
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users:
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alice:
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name: Alice Appleworth
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telephone: 123-456-7890
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bob:
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name: Bob Bananarama
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telephone: 987-654-3210
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And you want to print every user's name and phone number. You can loop through the elements of a hash using ``with_dict`` like this::
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tasks:
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- name: Print phone records
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debug: msg="User {{ item.key }} is {{ item.value.name }} ({{ item.value.telephone }})"
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with_dict: "{{users}}"
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.. _looping_over_fileglobs:
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Looping over Fileglobs
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``````````````````````
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``with_fileglob`` matches all files in a single directory, non-recursively, that match a pattern. It can
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be used like this::
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---
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- hosts: all
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tasks:
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# first ensure our target directory exists
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- file: dest=/etc/fooapp state=directory
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# copy each file over that matches the given pattern
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- copy: src={{ item }} dest=/etc/fooapp/ owner=root mode=600
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with_fileglob:
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- /playbooks/files/fooapp/*
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.. note:: When using a relative path with ``with_fileglob`` in a role, Ansible resolves the path relative to the `roles/<rolename>/files` directory.
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Looping over Parallel Sets of Data
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``````````````````````````````````
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.. note:: This is an uncommon thing to want to do, but we're documenting it for completeness. You probably won't be reaching for this one often.
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Suppose you have the following variable data was loaded in via somewhere::
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---
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alpha: [ 'a', 'b', 'c', 'd' ]
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numbers: [ 1, 2, 3, 4 ]
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And you want the set of '(a, 1)' and '(b, 2)' and so on. Use 'with_together' to get this::
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tasks:
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- debug: msg="{{ item.0 }} and {{ item.1 }}"
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with_together:
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- "{{alpha}}"
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- "{{numbers}}"
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Looping over Subelements
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````````````````````````
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Suppose you want to do something like loop over a list of users, creating them, and allowing them to login by a certain set of
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SSH keys.
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How might that be accomplished? Let's assume you had the following defined and loaded in via "vars_files" or maybe a "group_vars/all" file::
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---
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users:
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- name: alice
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authorized:
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- /tmp/alice/onekey.pub
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- /tmp/alice/twokey.pub
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mysql:
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password: mysql-password
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hosts:
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- "%"
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- "127.0.0.1"
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- "::1"
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- "localhost"
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privs:
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- "*.*:SELECT"
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- "DB1.*:ALL"
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- name: bob
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authorized:
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- /tmp/bob/id_rsa.pub
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mysql:
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password: other-mysql-password
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hosts:
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- "db1"
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privs:
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- "*.*:SELECT"
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- "DB2.*:ALL"
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It might happen like so::
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- user: name={{ item.name }} state=present generate_ssh_key=yes
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with_items: "{{users}}"
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- authorized_key: "user={{ item.0.name }} key='{{ lookup('file', item.1) }}'"
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with_subelements:
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- users
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- authorized
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Given the mysql hosts and privs subkey lists, you can also iterate over a list in a nested subkey::
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- name: Setup MySQL users
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mysql_user: name={{ item.0.user }} password={{ item.0.mysql.password }} host={{ item.1 }} priv={{ item.0.mysql.privs | join('/') }}
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with_subelements:
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- users
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- mysql.hosts
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Subelements walks a list of hashes (aka dictionaries) and then traverses a list with a given (nested sub-)key inside of those
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records.
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Optionally, you can add a third element to the subelements list, that holds a
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dictionary of flags. Currently you can add the 'skip_missing' flag. If set to
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True, the lookup plugin will skip the lists items that do not contain the given
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subkey. Without this flag, or if that flag is set to False, the plugin will
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yield an error and complain about the missing subkey.
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The authorized_key pattern is exactly where it comes up most.
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.. _looping_over_integer_sequences:
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Looping over Integer Sequences
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``````````````````````````````
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``with_sequence`` generates a sequence of items in ascending numerical order. You
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can specify a start, end, and an optional step value.
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Arguments should be specified in key=value pairs. If supplied, the 'format' is a printf style string.
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Numerical values can be specified in decimal, hexadecimal (0x3f8) or octal (0600).
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Negative numbers are not supported. This works as follows::
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---
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- hosts: all
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tasks:
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# create groups
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- group: name=evens state=present
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- group: name=odds state=present
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# create some test users
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- user: name={{ item }} state=present groups=evens
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with_sequence: start=0 end=32 format=testuser%02x
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# create a series of directories with even numbers for some reason
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- file: dest=/var/stuff/{{ item }} state=directory
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with_sequence: start=4 end=16 stride=2
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# a simpler way to use the sequence plugin
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# create 4 groups
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- group: name=group{{ item }} state=present
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with_sequence: count=4
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.. _random_choice:
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Random Choices
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``````````````
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The 'random_choice' feature can be used to pick something at random. While it's not a load balancer (there are modules
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for those), it can somewhat be used as a poor man's loadbalancer in a MacGyver like situation::
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- debug: msg={{ item }}
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with_random_choice:
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- "go through the door"
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- "drink from the goblet"
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- "press the red button"
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- "do nothing"
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One of the provided strings will be selected at random.
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At a more basic level, they can be used to add chaos and excitement to otherwise predictable automation environments.
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.. _do_until_loops:
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Do-Until Loops
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``````````````
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.. versionadded: 1.4
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Sometimes you would want to retry a task until a certain condition is met. Here's an example::
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- action: shell /usr/bin/foo
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register: result
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until: result.stdout.find("all systems go") != -1
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retries: 5
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delay: 10
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The above example run the shell module recursively till the module's result has "all systems go" in its stdout or the task has
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been retried for 5 times with a delay of 10 seconds. The default value for "retries" is 3 and "delay" is 5.
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The task returns the results returned by the last task run. The results of individual retries can be viewed by -vv option.
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The registered variable will also have a new key "attempts" which will have the number of the retries for the task.
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.. _with_first_found:
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Finding First Matched Files
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```````````````````````````
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.. note:: This is an uncommon thing to want to do, but we're documenting it for completeness. You probably won't be reaching for this one often.
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This isn't exactly a loop, but it's close. What if you want to use a reference to a file based on the first file found
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that matches a given criteria, and some of the filenames are determined by variable names? Yes, you can do that as follows::
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- name: INTERFACES | Create Ansible header for /etc/network/interfaces
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template: src={{ item }} dest=/etc/foo.conf
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with_first_found:
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- "{{ansible_virtualization_type}}_foo.conf"
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- "default_foo.conf"
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This tool also has a long form version that allows for configurable search paths. Here's an example::
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- name: some configuration template
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template: src={{ item }} dest=/etc/file.cfg mode=0444 owner=root group=root
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with_first_found:
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- files:
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- "{{inventory_hostname}}/etc/file.cfg"
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paths:
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- ../../../templates.overwrites
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- ../../../templates
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- files:
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- etc/file.cfg
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paths:
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- templates
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.. _looping_over_the_results_of_a_program_execution:
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Iterating Over The Results of a Program Execution
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`````````````````````````````````````````````````
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.. note:: This is an uncommon thing to want to do, but we're documenting it for completeness. You probably won't be reaching for this one often.
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Sometimes you might want to execute a program, and based on the output of that program, loop over the results of that line by line.
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Ansible provides a neat way to do that, though you should remember, this is always executed on the control machine, not the local
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machine::
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- name: Example of looping over a command result
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shell: /usr/bin/frobnicate {{ item }}
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with_lines: /usr/bin/frobnications_per_host --param {{ inventory_hostname }}
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Ok, that was a bit arbitrary. In fact, if you're doing something that is inventory related you might just want to write a dynamic
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inventory source instead (see :doc:`intro_dynamic_inventory`), but this can be occasionally useful in quick-and-dirty implementations.
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Should you ever need to execute a command remotely, you would not use the above method. Instead do this::
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- name: Example of looping over a REMOTE command result
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shell: /usr/bin/something
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register: command_result
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- name: Do something with each result
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shell: /usr/bin/something_else --param {{ item }}
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with_items: "{{command_result.stdout_lines}}"
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.. _indexed_lists:
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Looping Over A List With An Index
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`````````````````````````````````
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.. note:: This is an uncommon thing to want to do, but we're documenting it for completeness. You probably won't be reaching for this one often.
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.. versionadded: 1.3
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If you want to loop over an array and also get the numeric index of where you are in the array as you go, you can also do that.
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It's uncommonly used::
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- name: indexed loop demo
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debug: msg="at array position {{ item.0 }} there is a value {{ item.1 }}"
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with_indexed_items: "{{some_list}}"
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.. _flattening_a_list:
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Flattening A List
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`````````````````
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.. note:: This is an uncommon thing to want to do, but we're documenting it for completeness. You probably won't be reaching for this one often.
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In rare instances you might have several lists of lists, and you just want to iterate over every item in all of those lists. Assume
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a really crazy hypothetical datastructure::
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----
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# file: roles/foo/vars/main.yml
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packages_base:
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- [ 'foo-package', 'bar-package' ]
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packages_apps:
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- [ ['one-package', 'two-package' ]]
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- [ ['red-package'], ['blue-package']]
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As you can see the formatting of packages in these lists is all over the place. How can we install all of the packages in both lists?::
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- name: flattened loop demo
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yum: name={{ item }} state=installed
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with_flattened:
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- "{{packages_base}}"
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- "{{packages_apps}}"
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That's how!
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.. _using_register_with_a_loop:
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Using register with a loop
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``````````````````````````
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When using ``register`` with a loop the data structure placed in the variable during a loop, will contain a ``results`` attribute, that is a list of all responses from the module.
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Here is an example of using ``register`` with ``with_items``::
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- shell: echo "{{ item }}"
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with_items:
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- one
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- two
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register: echo
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This differs from the data structure returned when using ``register`` without a loop::
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{
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"changed": true,
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"msg": "All items completed",
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"results": [
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{
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"changed": true,
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"cmd": "echo \"one\" ",
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"delta": "0:00:00.003110",
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"end": "2013-12-19 12:00:05.187153",
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"invocation": {
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"module_args": "echo \"one\"",
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"module_name": "shell"
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},
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"item": "one",
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"rc": 0,
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"start": "2013-12-19 12:00:05.184043",
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"stderr": "",
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"stdout": "one"
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},
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{
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"changed": true,
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"cmd": "echo \"two\" ",
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"delta": "0:00:00.002920",
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"end": "2013-12-19 12:00:05.245502",
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"invocation": {
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"module_args": "echo \"two\"",
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"module_name": "shell"
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},
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"item": "two",
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"rc": 0,
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"start": "2013-12-19 12:00:05.242582",
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"stderr": "",
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"stdout": "two"
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}
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]
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}
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Subsequent loops over the registered variable to inspect the results may look like::
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- name: Fail if return code is not 0
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fail:
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msg: "The command ({{ item.cmd }}) did not have a 0 return code"
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when: item.rc != 0
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with_items: "{{echo.results}}"
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.. _writing_your_own_iterators:
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Writing Your Own Iterators
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``````````````````````````
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While you ordinarily shouldn't have to, should you wish to write your own ways to loop over arbitrary datastructures, you can read :doc:`developing_plugins` for some starter
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information. Each of the above features are implemented as plugins in ansible, so there are many implementations to reference.
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.. seealso::
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:doc:`playbooks`
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An introduction to playbooks
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:doc:`playbooks_roles`
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Playbook organization by roles
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:doc:`playbooks_best_practices`
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Best practices in playbooks
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:doc:`playbooks_conditionals`
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Conditional statements in playbooks
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:doc:`playbooks_variables`
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All about variables
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`User Mailing List <http://groups.google.com/group/ansible-devel>`_
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Have a question? Stop by the google group!
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`irc.freenode.net <http://irc.freenode.net>`_
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#ansible IRC chat channel
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